Development devices including a developer carrier provided with multiple electrodes to which different voltages are applied are known.
For example, there are development devices that supply toner to latent images formed on latent image bearers, such as photoreceptors, to develop the latent image without bringing the toner into direct contact with the latent image bearer. An example of such development methods is to supply toner to the latent image bearer by causing the toner to hop and form clouds (i.e., toner clouds) on or around the developer carrier. The developer carrier used in this method includes multiple different types of electrodes arranged alternately at a predetermined pitch in the circumferential direction of the developer carrier, and an outer circumferential side of the electrodes is covered with a protective layer. Separate voltages that change differently from each other with time are applied to the different types of electrodes, thus generating electrical fields that change differently from each other with time between adjacent electrodes. Then, the electrical fields cause the toner to hop between the adjacent electrodes and form toner clouds, which is a phenomenon hereinafter referred to as “toner flare” or “a flare state”. Thus, the toner forms clouds around the outer circumferential surface of the developer carrier.
In this method, to prevent the toner from adhering to the outer circumferential surface of the developer carrier and secure the hopping of toner, it is important to maintain a proper relation between a force F1 applied to the toner by the electrical field (hereinafter “flare electrical field”) formed between the different types of electrodes, adjacent to each other, and a force of adhesion between the toner and the outer circumferential surface of the developer carrier (adhesion force F2). If the force F1 is smaller than the adhesion force F2, the toner adheres to the outer circumferential surface of the developer carrier, thus failing to hop. By contrast, when the force F1 is greater than the adhesion force F2, the toner can hop. As the difference between the force F1 and the adhesion force F2 increases, the flare state becomes more stable. Although a stable flare state can be attained by increasing the force F1 to increase the difference, it is necessary to generate a greater flare electrical field on the outer circumferential surface of the developer carrier.
JP-2007-133388-A proposes a roller-shaped developer carrier including two types of electrodes (e.g., A-type electrode and B-type electrode) arranged concentrically with the developer carrier to generate the flare electrical fields. Two types of electrodes are shaped like combs and arranged in the circumferential direction of the developer carrier so that tooth portions of the A-type electrode are interposed between the two tooth portions of the B-type electrode. Then, different voltages are applied to the two types of electrodes to cause the toner to hop between the tooth portions, thereby attaining toner flare.
Additionally, JP-2008-116599-A proposes a roller-shaped developer carrier including three types of electrodes to generate the flare electrical fields. In this developer carrier, whereas first and second electrodes are arranged concentrically with the developer carrier, the third electrode is positioned outside the first and second electrodes, closer to the outer surface of the developer carrier. In this developer carrier, different voltages are applied to the three different electrodes to cause the toner to hop between the adjacent electrodes, thereby attaining the toner flare.
Although the out circumferential surface of the developer carrier is typically covered with an electrically insulative layer, the insulative surface layer is charged to the polarity opposite the polarity of the toner by triboelectric charging with the toner. Accordingly, the absolute value of the surface potential of the developer carrier increases as image development is repeated. As a result, development electrical fields change in size gradually, causing the image quality to fluctuate.